1. Filed of the Invention
The present invention relates to an image forming apparatus having a function of adding a predetermined pattern to a color image.
2. Related Background Art
Various countermeasures have been proposed to prevent counterfeits of securities and the like. As one of the techniques, the present applicant proposed a method of superposing a machine number unique to an image forming apparatus on an image in an almost unnoticeable color so as to specify an image forming apparatus used (U.S. patent application Ser. No. 08/009735).
Assume that a counterfeit of securities is formed using an image forming apparatus having a superposing function. In this case, when the machine number unique to this image forming apparatus is read from the counterfeit, the image forming apparatus used can be specified. This can serve as a good evidence to track down a counterfeiter.
When the image forming characteristics of the image forming apparatus vary depending on environmental changes, the machine number unique to the image forming apparatus, which is superposed on an image, cannot be read, or an image abnormality may occur.
For example, under a high-temperature, high-humidity condition, when the image reproduction capacity of the image forming apparatus may be decreased and high image quality tends not to be obtained, the superposed machine number unique to the image forming apparatus cannot be reproduced or read.
To the contrary, under a condition in which the image reproduction capacity of the image forming apparatus can be increased, the superposed machine number unique to the image forming apparatus is visually noticed on the image, thereby resulting in poor image quality.
According to the conventional technique described above, when the image forming apparatus is used for a long period of time to degrade the durability of a photosensitive body or the durability of a developer, the superposed machine number unique to the image forming apparatus cannot be read or identified or may be visually noticeable.
In the above method, a signal unique to the image forming apparatus is uniformly superposed regardless of types of images to be generally formed on recording media. For example, a color image is often formed on a film-like material (recording medium) such as an OHP film and is projected using a projector. Conventionally, a signal (superposition signal) to be superposed is a signal suitable for a reflective recording medium such as PPC paper, thermal paper, or coated paper. The recorded signal cannot be visually observed with a naked eye by normal reflected light. The recorded signal can be read by only a specific reader. However, a material such as an OHP film is projected using light transmitted therethrough, and a superposition portion becomes a shade and is clearly observed on a screen, resulting in inconvenience.
A machine number is also greatly influenced by the color of the background of a material on which a color image is formed. A yellow superposition signal which is unnoticeable on white paper is easily noticed on paper having a color complementary to yellow, often resulting in image defects.